"At the time I was not sure who the Big Man was but I suspected that the Big Man was Nathan Tinkler," Mr Thomson said.

"This was common parlance for Nathan Tinkler as he was both literally and metaphorically a big man.

"I first became aware from Mike Gallacher that there was a large donor who was going to contribute to the Newcastle campaign."

The funds were for the 2011 NSW election campaign and helped pay the $10,000 to $12,500 monthly wage of a media adviser.

The real identity of the Big Man was fiercely challenged for hours by Mr Moses.

"Mr Gallacher never said to you that Mr Tinkler was the big man ... do you accept that?," he put to Mr Thomson.

"I certainly don't reject it," Mr Thomson replied.

"You were referring to Mr O'Farrell, weren't you?" Mr Moses asked.

"Absolutely not," Mr Thomson said.

While much of the day was focused on the Big Man and his big wad of cash, there were other revelations.

Earlier, Mr Thomson said he was involved in Liberal Party plans to subvert electoral donation laws.

The ICAC heard he was a member of a group who concocted a scheme to have campaign service providers invoice third parties, who were prohibited from making political donations.

"If, for example, a media company did work for the campaign, that invoice could be sent through a prohibited donor," ICAC Commissioner Megan Latham inquired.

"I was certainly involved in it," Mr Thomson said, adding there were other Liberal ploys like the "sale of artwork for inflated prices".

The ICAC heard Mr Thomson stashed $20,000, given to him in $10,000 instalments by property developer and now Newcastle mayor Jeff McCloy and another property developer Hilton Grugeon, in a cupboard at his home.

Mr Owen, the former deputy commander of Australian forces in Afghanistan and Iraq, resigned from parliament on Wednesday after lying to the ICAC the day before about taking an illegal $10,000 political donation.